Due to its outstanding physical chemical and electrical properties silicon carbide is used inter alia as a semiconductor substrate material for power electronics semiconductor components, for radio frequency components and for a variety of special light emitting semiconductors components. Bulk SiC crystals with ideally pure and defect-free quality are required as a basis for these products.
As this is known in the art, bulk SiC crystals are generally produced by means of physical vapor deposition techniques, in particular using a sublimation method. Temperatures of more than 2000° C. are required therefore. Physical vapor transport (PVT) is essentially a sublimation and re-condensation process, in which a source material and a seed crystal are placed inside a growth furnace in such a way that the temperature of the source material is higher than that of the seed, so that the source material sublimes and the vapor species diffuse and deposit onto the seed to form single crystals. PVT growth techniques have proven to produce commercially usable SiC substrate materials, however, they suffer from the fact that the growth rate is rather slow with typically being in the range of some 100 μm/h. Thus, there is a need for a process and a belonging apparatus to grow SiC crystals more rapidly with unimpaired characteristics.
FIG. 4 shows a schematic view of a typical PVT growth cell, wherein PVT growth of a SiC single crystal is carried out in a graphite crucible 202 sealed with a graphite lid and loaded with a sublimation source 204 disposed at the bottom of the crucible 202. A single crystal seed 206 is arranged at the crucible top. A thermal insulation material 208 surrounds the crucible 202 and is only open in the region of a heat dissipation channel 110 which generates the temperature gradient which is necessary for re-condensation.
The sublimation source 204 is usually a polycrystalline SiC grain or powder synthesized in a separate process. The loaded crucible 202 is placed inside a growth chamber where it is surrounded by the thermal insulation 208.
Inductive or resistive heating (not shown in the figure) is used to bring the crucible 202 to a suitable temperature, generally between 2000° C. and 2400° C. for the PVT growth of a SiC single crystal on the SiC single crystal seed 206. The growth chamber may for instance be made of fused silica, and an RF coil is positioned with respect to the crucible 202 such that during growth of the single crystal the temperature of the sublimation source 204 is maintained higher than the temperature of the seed crystal 206 (typically with a difference of 10 to 200K).
Upon reaching a suitably high temperature, the sublimation source 204 vaporizes and fills the crucible 202 with a vapor of silicon, Si2C and SiC2 molecules. The temperature difference between the sublimation source 204 and the seed crystal forces the vapor to migrate and to condense on the seed crystal 206, thereby forming a single crystal boule. In order to control the growth rate, PVT growth is typically carried out in the presence of a small pressure of inert gas, usually between several and 100 Torr.
The published US patent application US 2012/0103249 A1 gives examples of such conventional PVT growth systems as shown in FIG. 4. Additionally, this document also proposes to surround the seed crystal by a gas permeable membrane consisting of graphite, for forming an envelope to create a quasi-closed vapor circulation space in the direct vicinity of the seed crystal.
However, as already mentioned, the growth rate of a SiC single crystal on the seed crystal 206 is typically around 100 μm/h and therefore, the growth of suitably large boules takes several days.
Consequently, it is suggested by several publications to grow more than one SiC single crystal boule simultaneously.
For instance, European patent EP 1099014 B1 proposes a device for producing a silicon carbide (SiC) single crystal which contains a crucible having a storage region for holding a stock of solid SiC and having a crystal region for holding two adjacent SiC seed crystals. An insert made from glassy carbon is disposed in the crucible. In the method, solid SiC is sublimed as a result of the stock being heated and SiC in the gas phase is generated, which is conveyed to the SiC seed crystals, on which it grows as an SiC single crystal. A heat flux is controlled by an insert made from glassy carbon. However this known arrangement firstly does not provide the rotationally symmetric temperature field which is necessary for avoiding defects that grow into the SiC boule from the peripheral regions. Secondly, a significant amount of costs is generated by the expensive highly pure crucible materials and the costs for the surrounding apparatus. Thus this known device causes a significantly enlarged diameter compared to the arrangement shown in FIG. 4 and therefore involves higher costs for the higher amount of SiC source material, crucible dimensions and apparatus geometry, possibly even overcompensating the cost reduction by simultaneously growing two bulk crystals.
Moreover, the German published patent application DE 19833755 A1 discloses a PVT growth system, wherein in a common reaction chamber, a plurality of seed crystal holders are arranged one above the other. In particular, the seed crystal holders are arranged along a symmetry axis of the growth apparatus and are separated by perforated screens. However, a problem with this known arrangement is that a sufficiently effective heat dissipation channel can only be provided for the topmost seed crystal and that therefore the crystal growth results for the inner seed crystals are not satisfactory.
On the other hand, U.S. Pat. No. 7,279,040 B1 proposes an arrangement for growing ZnO single crystal boules, where the seed crystals are arranged at opposing ends of a common growth chamber. The ZnO source material is placed in an inner region of the growth chamber. For this end, a source support is provided in the middle of the crucible. However, providing the source support in the arrangement according to this document leads to a lack of symmetry which will impair the characteristics of the simultaneously grown bulk crystals.